Characteristics and inheritance of the leaf mutation ins

Smirnova, O.G.                                                       Inst. of Cytology and Gen., Russian Acad. of Sci.

                                                                                                                         Novosibirsk, Russia

      Lamprecht was the first to describe insecatus leaves (gene symbol ins) (1). He studied a pea line with dissected tips of the first pair of leaflets. A central vein of the incised leaflet is overdeveloped into a small tendril (Fig.1, left).

      In contrast to homeotic pea leaf development mutants afila and tendril-less(tl) the mutation of gene ins affects only a few compound pea leaves. We have observed the insecatus phenotype in several lines and determined the late flowering the insecatus phenotype appearance 2-3 nodes below the first flowering node. Following initial formation of insecatus leaves, they may also occur at the next 3-4 nodes. Thus, insecatus leaves are formed in the middle part of a plant whereas lower and upper nodes of such plants have normal leaves. Unknown developmental and/or physiological mechanisms led to this pattern of insecatus leave phenotype distribution.  In our collection we have not observed the insecatus phenotype in early flowering pea lines. In order to analyze the influence of a flowering node on formation of insecatus character the F₂ hybrids differed in lf alleles were studied (Table 1). The lines OK7, OK14 (kindly provided by Dr. O.E. Kosterin) and WL1325 flower at node 6, counting the first scale leaf as node 1, and have lf-a, Ins genotype. The lines WL1751 (Ins), WIR2521 (ins) WIR319 (ins) belong to the late flowering class.

      Data for all late plants of five crosses presented in Table 1 were used to obtain a correlation between the first flowering node and the first insecatus node. The correlation coefficient is r=0.83. These results show a regular trend: the higher the first flowering node; the higher the first insecatus node.

      The interaction of different leaf development mutants is of great interest because of the control of compound leaf development. We described the tl^w gene influence on exhibition of insecatus phenotype. Homozygous tl^w/tl^w, ins/ins plants had instead of overdeveloped central vein a small leaflet (Fig. 1, middle). In heterozygous Tl/tl^w plants the overdeveloped central vein looked like a flat tendril (Fig. 1, right). Thus, a form of central vein of insecatus plants is completely defined by gene tl. The gene ins itself is responsible for extension of central vein beyond the edge of the blade and more or less symmetric leaflet dissection on both sides of this vein.

      Lamprecht established a single recessive gene inheritance for insecatus leaves (1). We have analyzed five hybrid combinations differing for ins alleles. In our experiments all F₁ plants had wild type phenotype. F₂ plants with any degree of dissection were identified as a recessive homozygous class. With such an approach the number of plants in different phenotypic classes is in a good accordance with the theoretically expected 3:1 ratio for the monogenic recessive control of insecatus leaves. 

      Encouraged by the good 3:1 fit for Ins – ins segregation in F₂ we have studied segregation of ins with marker genes. The fact that ins was in repulsion phase relative to the most of marker genes made the interpretation of segregations data more complicated because of large standard errors for the determination of genetic distances.  There was no linkage between ins and any of 29 marker genes listed here: d, o, i, ar, M, b, gl, v, gp, te, Fs, U^st, Pl, r, tl^w, bt, le, Np, fas, k, wb, a, lf, blb(2), oh, st, SCA(3), PSP3, PSP7 (the two last code proteins separated by electrophoresis in acetic acid). Lamprecht also failed to locate ins (1). In his experiments the initial pea line has the gene ins with precise expressivity, while in the F₂ progeny the plants appeared with insignificant leaflet dissection. The continuous variability from strong to insignificant dissection created complexities at phenotypic classification.

      Our experiments also show there are genes modifying the gene ins expression. Two lines WIR319 and WL6 each with the "normal" insecatus expressivity (5-6 incised leaflets per a plant) have been crossed. All F₂ offspring had dissected leaflets but the amount of insecatus leaflets varied very wide from plant to plant. As all F₂ plants were recessive homozygotes ins/ins and were growing in the same conditions of a greenhouse, it is obvious that a genetic background of a plant influences on an insecatus gene expression.

In some case the mutation ins, like ins2 (4, 5), displays dominance. In the cross WL1238 x WIR2521 we have obtained 8 F₁ plants: 2 F₁ with WT and 6 F₁ with mutant insecatus phenotype. Seven F₁ plants were used to generate the test-cross WL1238 x (WL1238xWIR2521) (Table 2). Test-cross data reveal good 1:1 segregation. But varying dominance of insecatus phenotype in the same cross makes genetic mapping of ins gene difficult. For further genetic analysis, we will need to find ins alleles with better penetrance and stable expression.

1.   Lamprecht, H.  1959.  Agri Hort. Genet. 17: 26-36.

2.   Kosterin, O.E., Rozov, S.M.  1993.  Pisum Genetics 25: 27-31.

3.   Smirnova, O.G., Rozov, S.M., Kosterin, O.E. and Berdnikov, V.A.  1992.  Plant Sci. 82: 1-13.

4.   Berdnikov, V.A., Gorel, F.L., Bogdanova, V.S. and Kosterin, O.E.  2000.  Pisum Genetics 32: 9-12.

5.   Berdnikov, V.A. and Gorel, F.L.  2001.  Pisum Genetics 33: 26.